Despite its common name, the Least Killifish is actually one of the smallest livebearing fish in the world, belonging to the Poeciliidae family, not the killifish family. Native to the southeastern United States, they are incredibly peaceful and perfect for nano tanks. Unlike most livebearers, they exhibit 'superfoetation', meaning females drop 1-2 fry every few days rather than a large single brood.
At a Glance
Tank Requirements
Water Parameters
Temperament & Compatibility
Available Varieties
Each variety inherits the species defaults above. Only differences are highlighted.
Female
Females are significantly larger and bulkier than males, featuring a distinct dark spot near their anal fin (the gravid spot). They are the dominant presence in a colony.
Same care requirements as the base species
Also known as: Female Least Killifish, Standard
Male
Males are exceptionally tiny, ranking among the smallest known vertebrates. They are highly active, constantly courting females, and possess a modified anal fin called a gonopodium.
Differs in 2 traits from base species
Also known as: Male Least Killifish
Least Killifish (Heterandria) Care Summary
The Least Killifish (Heterandria), scientifically known as Heterandria formosa, belongs to the Livebearers family group. Adults reach a maximum size of approximately 3.5 cm and have a fusiform body shape with a fast growth rate. Classified as peaceful, this species primarily occupies the surface, mid-level, and bottom zones of the aquarium and produces a low level of biological waste.
To thrive, the Least Killifish (Heterandria) requires a brackish tolerant aquarium with a minimum volume of 19 litres and a tank length of at least 30 cm. Water temperature should be maintained between 18°C and 27°C, with a pH range of 7–8 and a general hardness of 5–20 dGH. A low water flow is preferred. To replicate their natural habitat, the tank should include plants - densely covered and plants - floating.
This species should be kept in groups of at least 3 to reduce stress and encourage natural behaviour. This species is available in 2 varieties: Female and Male. Each may differ slightly in size, temperament, or care needs, so it is worth checking the specific requirements of the variety you plan to keep.
Similar Species
Clown Killifish
Epiplatys annulatus
A striking nano killifish resembling a tiny rocket. Native to West African swamps, they have distinct black and yellow vertical bands. They are strictly surface dwellers (top-level) and are non-annual, meaning they live longer than annual killifish (2-3+ years). They are notorious jumpers and require a tight lid.
Gardner's Killifish
Fundulopanchax gardneri
A stunning, semi-annual killifish native to Nigeria and Cameroon. Known for being relatively hardy and an excellent 'beginner' killifish. They are notorious jumpers and require a tightly fitting lid. While they can be kept in community tanks with similarly sized, fast-moving fish, males can be boisterous and territorial.
Dwarf Pea Puffer
Carinotetraodon travancoricus
The Dwarf Pea Puffer is a tiny, fully freshwater pufferfish native to India. Despite their cute appearance, they are fierce micropredators and notorious fin-nippers. Modern husbandry standards strongly dictate they are highly social and must be kept in species-only shoals of 6 or more in heavily planted aquariums to disperse aggression and reduce stress. They require a steady diet of small snails and live/frozen foods.
Scarlet Badis
Dario dario
One of the smallest known aquarium fish, resembling a tiny jewel. Males display brilliant vertical red and iridescent blue stripes. While beautiful, they are known for being extremely picky eaters, often refusing dry foods and requiring live or frozen micro-foods (Daphnia, Baby Brine Shrimp). Males act like tiny Cichlids and will stake out territories.
Florida Flagfish
Jordanella floridae
A robust, deep-bodied killifish native to Florida. Unlike most killifish, they are primarily herbivores and are renowned for their ability to eat Hair Algae and sometimes Black Beard Algae. They are sub-tropical and thrive in unheated aquariums. They can be fin-nippers, especially if underfed or kept with long-finned tank mates.
Bumblebee Goby (Brackish)
Brachygobius xanthozonus
A miniature, bottom-dwelling goby famous for its striking black and yellow stripes. While they are often sold as freshwater fish and can survive in it temporarily, they are highly susceptible to fungal infections in fresh water and truly require a brackish setup (SG 1.002-1.005) to thrive long-term. They are notoriously picky eaters, often ignoring flakes in favor of live or frozen foods (like bloodworms or brine shrimp).